Supervisory circuits are used to monitor power supply voltages and switch to or from a backup power source depending on the acceptability of the monitored voltage. Supervisory circuits are used in microprocessors, digital equipment, and various other electronic equipment and systems. A supervisory circuit commonly contains a switching circuit that is used to switch the load between power derived from the power supply and power derived from a battery, and back again. This switching circuit must furnish a high degree of isolation between switched power sources, so that for example the battery is not loaded by any part of the power supply circuitry when running off the battery. Circuit elements that may provide inadequate isolation during low power supply voltage conditions are the main switching circuit and other functional elements such as logic gates.